An image sensor is applied to an automatic navigation system, which is incorporated in a vehicle such as an automobile, for monitoring the forward view of the vehicle so that the automatic navigation system can control such operations of the vehicle as steering to follow a white guiding line which is provided along the roadway by capturing the guiding line with the image sensor or braking appropriately in response to changes in the vehicle's forward environment which may include other vehicles preceding ahead and other objects. In such monitoring, the performance required of the image sensor includes a capacity to detect an obstacle 200 meters or more away ahead of the vehicle. This is especially true for the vehicle to drive on a highway. For this reason, the automatic navigation system has required a camera which incorporates a high-resolution imaging device.
A conventional method for such monitoring has made use of two general television cameras. While one camera equipped with a telephoto lens is used for monitoring distant views in detail, the other camera with an ordinary lens is used for monitoring close views with a wide angle. Another method has utilized the relatively high resolution of a Hi-Vision camera, and views are monitored collectively without any distinction whether they are distant from or close to the vehicle.
However, these methods of the prior art have had problems inherent to the types of camera which are used. One of the problems is that any of the cameras which may be applied as such sensor is so expensive that the hardware of the automatic navigation system which incorporates any of these cameras has been rendered impracticably expensive. Furthermore, if a high-resolution camera such as a Hi-Vision camera is applied as the image sensor for monitoring the forward view of a vehicle, then this image sensor inherently takes in a large amount of information for image formation without any distinction whether the views captured are distant or near. This presents another problem because the images captured by the image sensor can easily overload the image processing capacity of the automatic navigation system. Because of the characteristic of high resolution, Hi-vision cameras are suitable for monitoring distant views. However, when they are used for monitoring near views with the same resolution, they tend to include details which are not really necessary for the object recognition of the automatic navigation system, and the load of image processing required for the recognition of the objects in the views is quite sizable.
It is not necessary for the system to monitor all objects at a constant high resolution. Some objects need to be monitored at a higher resolution. However, it all depends on the condition. For example, in the case shown in FIG. 1 where a camera is mounted on the front of a vehicle to monitor the forward view, the upper portions of the images which are being monitored generally include distant views of the roadway while the lower portions include near views thereof. Therefore, if the upper portions of the images are captured at a relatively high resolution while the lower portions are captured at a relatively low resolution, then the load of data to be processed or the information extracted from the images in the image processing can be effectively reduced or lightened without jeopardizing the recognition of the condition of the roadway. By realizing this feature for the image sensor, a camera for monitoring the forward view of a vehicle can be produced relatively inexpensively.